# std::contiguous_iterator

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Defined in header `<iterator>` |  |
template< class I > concept contiguous_iterator = std::random_access_iterator<I> && std::derived_from</*ITER_CONCEPT*/<I>, std::contiguous_iterator_tag> && std::is_lvalue_reference_v<std::iter_reference_t<I>> && std::same_as< std::iter_value_t<I>, std::remove_cvref_t<std::iter_reference_t<I>> > && requires(const I& i) { { std::to_address(i) } -> std::same_as<std::add_pointer_t<std::iter_reference_t<I>>>; }; |  | (since C++20)

The `contiguous_iterator` concept refines `random_access_iterator` by providing a guarantee the denoted elements are stored contiguously in the memory.

### Iterator concept determination

Definition of this concept is specified via an exposition-only alias template /*ITER_CONCEPT*/.

In order to determine /*ITER_CONCEPT*/<I>, let ITER_TRAITS<I> denote I if the specialization std::iterator_traits<I> is generated from the primary template, or std::iterator_traits<I> otherwise:

- If ITER_TRAITS<I>::iterator_concept is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
- Otherwise, if ITER_TRAITS<I>::iterator_category is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
- Otherwise, if std::iterator_traits<I> is generated from the primary template, /*ITER_CONCEPT*/<I> denotes std::random_access_iterator_tag.
- Otherwise, /*ITER_CONCEPT*/<I> does not denote a type and results in a substitution failure.

### Semantic requirements

Let `a` and `b` be dereferenceable iterators and `c` be a non-dereferenceable iterator of type `I` such that `b` is reachable from `a` and `c` is reachable from `b`. The type `I` models `contiguous_iterator` only if all the concepts it subsumes are modeled and:

- std::to_address(a) == std::addressof(*a),
- std::to_address(b) == std::to_address(a) + std::iter_difference_t<I>(b - a), and
- std::to_address(c) == std::to_address(a) + std::iter_difference_t<I>(c - a).

### Equality preservation

Expressions declared in requires expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

### Implicit expression variations

A requires expression that uses an expression that is non-modifying for some constant lvalue operand also requires implicit expression variations.

### Notes

`contiguous_iterator` is modeled by every pointer type to complete object type.

Iterator types in the standard library that are required to satisfy the LegacyContiguousIterator requirements in C++17 are also required to model `contiguous_iterator` in C++20.

### See also

random_access_iterator (C++20) | specifies that a `bidirectional_iterator` is a random-access iterator, supporting advancement in constant time and subscripting (concept)

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